The present invention relates generally to the field of semiconductors and more particularly to a highly conductive semiconductor layer having two or more impurities.
Semiconductors, either single element or compound (e.g., III-V) semiconductors, are widely used in integrated circuits. For example, Gallium Arsenide (GaAs) semiconductors are widely used in low-noise, high-gain, weak-signal amplifying devices. The useful properties of semiconductors depend not only on the particular semiconductor that forms the crystal, but also on the dopants that are incorporated into the crystal lattice.
Semiconductor devices require the use of highly conductive layers. The use of impurities or dopants contributes to the hole and electron charge carriers that are responsible for the electronic properties of the crystals. When excess electrons are generated, the impurity is considered a xe2x80x9cdonorxe2x80x9d or xe2x80x9cn-typexe2x80x9d dopant. When excess holes are generated, the impurity is considered an xe2x80x9cacceptorxe2x80x9d or xe2x80x9cp-typexe2x80x9d dopant.
More specifically, depending on the valence of the impurity and the lattice site, the conductivity of a semiconductor layer depends upon (i) the number of electrons (or holes); (ii) the electron (or hole) mobility; and (iii) the charge of the electron. Thus, there is a direct correlation between the level of impurity and the conductivity of the semiconductor layer. Thus, in order to create a highly conductive layer, the density of the impurity must also be very high.
Importantly, however, when any single impurity is added in very high concentrations in an attempt to maximize conductivity, degradation problems occur. Such problems depend on the particular dopant used, and include but are not limited to auto compensation, diffusion, strain, and other defects. For example, when a Beryllium (Be) dopant is used above a particular concentration, diffusion may occur that degrades device performance. As another example, excessive use of a Carbon (C) dopant likely causes auto-compensation that degrades device performance. Throughout this specification, the term xe2x80x9cdegradation concentrationxe2x80x9d is used to describe the concentration at which a particular impurity begins to create detrimental effects or degrade semiconductor performance. In other words, each impurity currently used to create conducting layers has some maximum acceptable concentration before the layer is degraded in some manner. See, for example, Doping in Semiconductors, E. F. Schubert, University Press, 1993, herein incorporated by reference.
To date, attempts to reach beyond the accepted degradation concentration for particular dopants have focused on manufacturing techniques to reduce the detrimental effects. For example, attempts to overcome the degradation concentration for Be have focused on reducing the layer temperature during manufacture. As another example, to reduce auto-compensation, strict control is maintained on the ratio of the semiconductor elements. Such techniques often add to the manufacturing time and costs, while providing only marginal gains in the conductivity of the semiconductor layer. Furthermore, such techniques focus on minimizing rather than avoiding or eliminating the potential detrimental effects.
There is a need in the art for a highly conductive semiconductor without the aforementioned degradation problems.
The present invention provides a novel approach to the creation of a highly conductive semiconductor layer. As noted above, prior attempts to create highly conductive layers have focused on minimizing rather than eliminating the potential detrimental effects. The present invention eliminates the detrimental effects of particular impurities by avoiding the use of individual impurities at densities beyond their respective degradation concentrations. The present invention combines two or more dopants, each at a level below the dopant""s degradation concentration, to provide a highly conductive layer.
More specifically, the present invention provides a semiconductor layer that includes at least two impurities. Each impurity is introduced at a level below its respective degradation concentration. In this manner, the two or more impurities provide an additive conductivity to the semiconductor layer at a level above the conductivity possible with any one of the impurities alone, due to the detrimental effects that would be created by increasing the concentration of any one impurity beyond its degradation concentration.
An additional aspect of the present invention is a semiconductor layer having two or more epitaxially grown impurities of the same carrier type. Preferably, the two or more impurities each has a smaller covalent radius than the layer atoms. More preferably, the two or more impurities are grown at substantially equivalent concentrations. The present invention also provides a method for creating a semiconductor layer having two or more impurities introduced during layer formation.
These and other aspects of the present invention as disclosed herein will become apparent to those skilled in the art after a reading of the following description of the preferred embodiments when considered with the drawings. The drawings are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the present invention.